Certain metal molybdate salts have been recognized for their ability to inhibit corrosion of ferrous substrates when incorporated as pigments in conventional and special coating formulations. Such metal molybdate pigments in addition to their excellent capacity to inhibit corrosion, also provide advantages over other more conventional corrosion inhibiting pigments in view of their neutral or white color, providing for increased latitude in the formulation of coating systems which are of comparatively light color. Additionally, the non-toxicity of such metal molybdate pigments enables unrestricted use thereof, including areas and articles subject to human contact.
In spite of the foregoing advantages, the use of molybdate corrosion inhibiting pigments has been restricted due to their relatively high cost in comparison to other more common corrosion inhibiting pigments including, for example, zinc chromate, basic lead chromate, dibasic lead phosphite and the like. Attempts to effect a reduction in the cost of corrosion inhibiting molybdate pigments by forming physical mixtures of the metal molybdate compounds with inert extender materials has not been satisfactory in view of the necessity of incorporating upwards of 50% by weight of the active metal molybdate salt to achieve the required corrosion inhibiting properties. Such physical mixtures, in many instances, are still not competitively priced relative to the other more commonly used corrosion inhibiting pigments and, in some instances, such physically extended pigments have exhibited less than optimum corrosion inhibiting performance.
The present invention overcomes the cost and processing disadvantages heretofore associated with molybdate corrosion inhibiting pigments by providing a method in which a particulated substantially inert and compatible carrier material is provided with an adherent coating of a controlled amount of selected metal molybdate salts, producing a non-toxic, colorless extended pigment which is competitively priced with other known corrosion inhibiting pigments and also provides for good dispersibility of the active molybdate constituent throughout a coating formulation.